This invention relates to metallic articles coated for improved environmental resistance, particularly hot corrosion resistance at elevated temperatures, and, more particularly, to such articles having a complex, graded coating interdiffused with the substrate.
The high temperature operating conditions of a gas turbine engine presented designers with a problem associated with component surface deterioration as a result of oxidation under such conditions. As a result, there have evolved a number of coating systems to protect the surfaces of those high temperature operating components such as turbine blades and vanes during operation in gas turbine engines. However, operation of such apparatus near or on bodies of salt water have presented additional problems associated with hot corrosion, the mechanism for which differs from oxidation.
It has been known for many years to improve environmental resistance of metallic articles through an aluminum or an aluminide coating. However, more recent efforts, which recognize the various types and interrelationships of surface oxides, have been reported. For example, U.S. Pat. No. 3,996,021--Chang, issued Dec. 7, 1976, recognizes the benefit of including in a surface coating the element Hf to provide HfO.sub.2 for improved coating life. In addition, U.S. Pat. No. 3,976,436--Chang, issued Aug. 24, 1976, describes the benefits of including such elements as Pt, Rh and Pd along with Al and Hf for improved environmental resistance. Additional benefit has been disclosed in U.S. Pat. Nos. 3,874,901 and 3,998,603--Rairden, III, through the use of an overlayer of aluminum. The disclosure of each of such patents is incorporated herein by reference.